by John Bacon and Donna Leinwand Leger, USA TODAY

by John Bacon and Donna Leinwand Leger, USA TODAY

The four buildings evacuated at Harvard after a Monday morning bomb threat disrupted finals week have reopened, the university said.

Thayer Hall, a freshman dorm, and three classroom buildings - Science Center, and Sever and Emerson Halls - reopened Monday afternoon, the school announced. Harvard Yard also reopened, the school said on its website.

The buildings were evacuated and final examinations in them were canceled when Harvard issued the alert shortly after 9 a.m. ET on its website, on Twitter and in e-mails. The alerts cited "unconfirmed reports of explosives at four sites on campus: Science Center, Thayer, Sever, and Emerson. Please evacuate those buildings now."

The school, in Cambridge, Mass., declined to reveal details of the threat. Faculty, staff and students were notified through the school's website, e-mail, voice mail and internal messaging system.

For several hours the school limited access to Harvard Yard, the historic center of campus, to those with Harvard ID cards.

"Out of an abundance of caution, the buildings have been evacuated while the report is investigated," the statement on the website said. "Harvard's focus is on the safety of our students, faculty and staff."

University and Cambridge police were being aided in the investigation by state and federal officials, the school said.

President Obama, a Harvard Law School graduate, had been briefed on the situation, the White House said.

The university tweeted that it was working to reschedule exams that had been scheduled for the affected buildings. Final exams have been taking place for several days, and scores of finals had to be canceled Monday.

Students Alexander Ryjik and Diego Abrahao said their professor was handing out exam booklets for their Politics of American Education class when the alert went out to leave Emerson Hall.

"I have a good guess somebody called it in so they wouldn't have to take an exam," Ryjik told the Associated Press. "It's frustrating because now the exam will have to be postponed."

A person making a fake bomb threat could face up to 20 years in prison, Boston criminal defense attorney Andrea Lance, a former Miami-Dade prosecutor, told USA TODAY.

"It's a crime against the public peace," Lance said. "You're not only disrupting people's lives, but you're using emergency services and you are causing fear.

"Police have to respond as if this is real. It takes a lot of resources," Lance said. "When the threat comes in, you don't know who is behind it. Is this a kid who wanted to get out of final exams or is this something like the Boston Marathon bombing last year?"

Jay Harris, dean of undergraduate education, told students gathered at Harvard's Annenberg Hall, the freshman dining facility, that many Monday finals had been canceled due to the scare, the Crimson reported.

"Following announcement, students erupt in applause," the Crimson tweeted.

The main campus of the school, among the nation's most prestigious, sits on 210 scenic acres along the banks of the Charles River. Harvard's student body includes about 6,700 undergraduates and another 14,500 graduate and professional students.